1. Technical Field
The present invention relates to a power generating device that derives electric energy from an electric charge when a piezoelectric material such as a piezoelectric device is transformed by an external force.
2. Related Art
When a piezoelectric material such as lead zirconate titanate (PZT), crystal (SiO2), or zinc oxide (ZnO) is transformed by an external force, electric polarization is caused inside the material, and positive and negative electric charge appears on the surface. Such a phenomenon is called a piezoelectric effect. A method of generating power is proposed in which, by using such a property of the piezoelectric material, a cantilever is vibrated so as to repeatedly weight the piezoelectric material, and an electric charge generated on the surface of the piezoelectric material is derived as electricity.
For example, by vibrating a metal cantilever which has a spindle arranged at its tip end and to which a thin plate of a piezoelectric material is attached, positive and negative electric charge alternately generated in the piezoelectric material in accordance with the vibration is taken out, thereby generating an AC current. A technique is proposed in which such an AC current is rectified by a diode, then stored in a capacitor, and taken out as electric power (JP-A-7-107752). In addition, a technique is proposed in which a DC current is acquired without incurring a voltage loss in a diode by closing the contact point only while positive charge is generated in the piezoelectric device (JP-A-2005-312269). By using such techniques, the power generating device can be miniaturized. Accordingly, for example, there are hopes for an application in which the power generating device is built into a small-sized electronic component instead of a battery cell, or the like.
However, according to the proposed general techniques, there is a problem that the acquired voltage is limited to the level of a voltage that is generated in accordance with electric polarization of the piezoelectric material. Accordingly, in most cases, a separate booster circuit is necessary, and there is a problem that it is difficult to decrease the size of the power generating device.